Is my master brake cylinder failing again?

93 22RE 2WD. About 2.5 years ago, my master brake cylinder failed. The symptom was when you push the brake pedal it feels normal but then as you hold it, it goes soft until the pedal is down to the floor. Thanks to advice from this sub, I picked up a replacement at O'Reilly, installed it, and it worked fine. Well, today, seemingly out of the blue, the brakes feel different. A little harder to press than they were yesterday, and also when you press down on the brake pedal you feel a sort of "notch" feeling that you push past. It's a small "notch." Sort of when you press a little harder on the brake pedal after you're already pressing it, you feel and actually hear it drop a bit all at once, and then it feels fairy normal again. It's definitely a little harder to push than it was, but it's still stopping just fine. I did a panic stop as a test from about 35mph on a straight road with no traffic, and the front brakes locked up as you'd expect them to.

So, what do you think? Is my store brand O'Reilly brake cylinder failing? Or my vacuum brake booster? The cylinder has a lifetime warranty so I can swap it out easily (though I probably should replace it with an OEM one).

I will add - I don't have a leak in the brake lines - my fluid level is fine, right where it was last time I checked it.